1. Field of the Invention
This invention relates to a gas discharge panel information read-out system, and more particularly to a system for reading out information in a gas discharge panel by detection of current flowing in discharge cells of the panel.
2. Description of the Prior Art
A gas discharge panel commonly referred to as a plasma display memory panel has heretofore been developed as an information display device. As is well-known in the art, the gas discharge panel has such a construction that electrodes arranged in a matrix form facing towards a gas discharge space are covered with a thin dielectric layer thereby to be insulated from the discharge gas. It is also well-known that the gas discharge panel is driven with an AC voltage and that, at a discharge point where a discharge is once produced in accordance with input information, the information is stored in the form of a wall charge. Accordingly, the gas discharge panel can be used not only as a display device but also as a memory device. In the case of employing the gas discharge panel of this kind as a display terminal of a graphic display system, it is necessary to read out information being displayed, since a high value is set on man-machine communication.
A variety of systems have been proposed for reading out the content of a display on the plasma display memory panel. For example, FIG. 1 shows one of them. In the plasma display memory panel indicated generally by PDM in FIG. 1, discharge cells are formed at intersecting points of electrodes y1 to yn and electrodes x1 to xm; an alternating sustain voltage is applied to each discharge cell through the electrodes Y1 to yn and X1 to Xm; and a discharge spot is produced in the discharge cell supplied with a write voltage. In the case of reading out the firing cell, i.e., a cell undergoing discharge, and thus at which the discharge spot is produced, for example, in the case of reading out a firing cell at the intersecting point of the electrodes y1 and x1, a terminal Y1 is grounded and a positive read voltage is applied to the electrode x1. At this time, the impedance of a saturable reactor L is held low and the electrode y1 is grounded through a diode D2 and the reactor L at terminal Y1.
If the read voltage is such as indicated by V in FIG. 2, a current flowing in the electrode y1 is as indicated by I. Since the discharge cell is a capacitive load, a charging current Ic flows at an early stage of rise of the voltage V and then a gas discharge current Ig flows when the voltage V has sufficiently risen up to its predetermined level. Further, a small backward discharge current Id flows at an instant of fall of the voltage V. This charging current Ic flows in all of the discharge cells regardless of whether they are firing or non-firing cells, and the gas discharge current Ig flows only in the firing cell, so that read-out is achieved by detecting the gas discharge current Ig.
To this end, the reactor L is controlled to increase its impedance at an instant when the gas discharge current Ig flows after flowing of the charging current Ic, thereby to cut off a current flowing in the diode D2. Consequently, since a diode D1 is connected in a backward direction, the gas discharge current Ig flows in a stray capacitance of the electrode y1 to raise its potential relative to the ground potential and the potential change is amplified by an amplifier AMP1 to derive a read-out output at a terminal YR1.
Assuming that the aforementioned charging current Ic starts to flow at an instant t0, reaches its maximum value at an instant t1 and then becomes substantially zero at an instant t2 and that the gas discharge current Ig flows following the charging current Ic and reaches its maximum value at an instant t3, it is necessary to control the reactor L at the instant t2 to cut off the current flowing in the diode D2. Since the time interval between t0 to t2 is as short as about less than 0.5.mu.S, control of the reactor L is difficult. Such timing control is also subject to the influence of the characteristics of the discharge cells and the rise-up, or leading edge characteristic of the read voltage, and hence is further difficult, introducing possibilities of inaccurate read-out.